Rolling elements

ABSTRACT

The pitting resistance of a gear is increased by hardening its tooth flanks through application of carburizing/quenching, bright hardening and induction hardening to a steel material capable of providing significantly improved softening resistance in tempering at a low temperature of 300 to 350° C. To this end, the steel material prepared so as to satisfy the relationship described by: 5≦4.3×Si(wt %)+7.3×Al(wt %)+3.1×V(wt %)+1.5×Mo(wt %)+1.2×Cr(wt %)×(0.45÷C(wt %)) is carburized such that the carbon concentration of its carburized surface layer is adjusted to 0.6 to 0.9 wt %; and the steel material is subjected to quenching and tempering at 300° C. or less subsequently to the carburization process, or alternatively the steel material is once cooled after the carburization process and then subjected to treatments of re-heating hardening and tempering at 300° C. or less so that a hardness of HRC 58 or more is ensured by the tempering process at 300° C.

CROSS REFERENCE TO RELATED APPLICATION

This application is a Divisional application of U.S. application Ser. No. 10/641,362, filed on Aug. 13, 2003.

TECHNICAL FIELD

The present invention relates to rolling elements produced by carburizing and quenching, bright hardening or induction hardening. More particularly, the invention relates to gears and rolling elements such as bearings, races and rollers, the gears being made from steel which provides significantly improved resistance to softening caused in low-temperature tempering at 300 to 350° C. and having high pitting resistance in the tooth flanks hardened by carburizing/quenching, bright hardening or induction hardening.

BACKGROUND ART

Up to now, gears produced by applying carburizing/quenching, carburizing/carbonitriding/quenching to SCr-based, SCM-based or SNCM-based low carbon steel have been commonly employed in the reducers of construction machines and earth-moving machines, since high contact fatigue strength (200 kgf/mm² or more) is considered to be an important factor. For ring gears used under the condition of comparatively low interface pressure (up to 150 kgf/mm²), gears produced by applying thermal treatment such as bright hardening or induction hardening to carbon steel or SMn-based middle carbon steel (0.45 to 0.6 wt % C) are used.

For the reducers of construction machines and earth-moving machines, less expensive gears having higher strength and higher resistance to interface pressure are required, in view of the recent tendency to higher output power and compactness.

Construction machines and earth-moving machines often stride obstacles such as rocks and structures during travelling and drill the obstacles while making a turn, and therefore, the gears of the reducer used for running and turning such machines receive impulsive load. This is a serious problem of damage to carburized quenched gears.

Bright-hardened or induction-hardened gears have higher toughness than carburized quenched gears, but are more likely to cause pitting or scuffing when they are used under high interface pressure such as noted above.

The invention is directed to overcoming the problem of the conventional carburized, quenched gears and induction-hardened gears which exhibit poor impact resistance when they have insufficient contact fatigue strength. Taking account of the fact that the contact fatigue strength of a gear used under a rolling/sliding contact condition highly depends on whether or not it has sufficient temper softening resistance against an increase up to 300° C. in the temperature of the tooth flanks during operation, the invention aims to provide various types of rolling elements such as carburizing and quenching gears for use under high interface pressure, which elements are made from a steel material to which a large amount of Al and/or Si (Al and Si can effectively increase resistance to softening caused by tempering at 300° C.) has been added and which elements have a temper hardness of HRC 58 or more after tempering at 300° C. The invention further aims to provide, through proper combined additions of Al and Ni to the above steel material, rolling elements which can exhibit high toughness in spite of their high hardness.

The present invention has been directed to overcoming the poor pitting resistance of gears hardened by bright hardening or induction hardening and therefore aims to provide inexpensive rolling elements such as high induction hardened gears which have been improved in temper softening resistance so as to have a temper hardness of HRC 54 or more at 300° C. and pitting resistance equivalent to that of carburized quenched gears, by more proper additions of Si, Al, V, Mn, Cr, Mo and Ni.

DISCLOSURE OF THE INVENTION

SNCM815, SCM420, SCr420, SMnB420 steels which had been subjected to carburizing and quenching were preliminarily tested in terms of rolling contact fatigue strength (pitting resistance) under the condition of rolling/sliding at interface pressures of 375 to 220 kgf/mm². As a result, it was found that the interface pressure at which pitting appeared after 10⁷ rotations was 210 kgf/mm² and the X-ray half value width of the martensitic phase of the outermost layer of the rolling contact surface in which pitting occurred under each pressure was reduced to 4 to 4.2°, and significant softening was observed at the outermost layer of the rolling contact surface.

An S55C carbon steel which has been subjected to quenching and tempering so as to have HRC 61 to 62 was preliminarily tested in terms of rolling contact fatigue strength at an interface pressure of 250 kgf/mm². As a result, it was found that the interface pressure at which pitting appeared after 10⁷ rotations was about 180 kgf/mm² and the X-ray half value width of the martensitic phase of the rolling contact surface in which pitting occurred under an interface pressure of 250 kgf/mm² was reduced to 3.6 to 4.2° similarly to the above-described carburized, case-hardened steels.

A preliminary test was also conducted on an eutectoid carbon steel (0.77 wt % C) to check its rolling contact fatigue strength. As a result, it was found that the interface pressure at which pitting appeared after 10⁷ rotations was about 230 to 240 kgf/mm² which was substantially the same as the rolling contact fatigue strength of the aforesaid carburized, case-hardened steels having substantially the same carbon content. It was also found that a decrease due to variation in rolling contact fatigue strength was observed in the carburized case-hardened steels because of the presence of an intergranular oxidation layer and a slack quenching layer in the rolling contact surface.

A preliminary test was conducted on an eutectoid carbon steel (0.82 wt % C), whose rolling contact surface had been subjected to induction hardening, to check its rolling contact fatigue strength and it was found that the interface pressure at which pitting appeared after 10⁷ rotations was about 260 to 270 kgf/mm² and this eutectoid carbon steel had higher rolling contact fatigue strength than the former eutectoid steel (0.77 wt % C) because of fine cementite particles dispersing in the martensitic phase of the rolling contact surface.

From the viewpoint of the dispersion of fine cementite particles, SUJ2 containing about 1.0 wt % C and 1.5 wt % Cr was quenched at 840° C. and then tempered to have HRC 62.5. The rolling contact fatigue strength of this steel was checked by a preliminary test and it Was found that the interface pressure at which pitting appeared after 10⁷ rotations was about 270 kgf/mm² which was approximately the same as that of the above eutectoid steel and that the X-ray half value width of the martensitic phase of the rolling surface in which pitting occurred under an interface pressure of 250 kgf/mm² was reduced to 4.2 to 4.50 similarly to the carburized, case-hardened steels described above.

Further, carbon steels having a carbon content of 0.46, 0.55, 0.66, 0.77 and 0.85 wt % respectively were quenched from a temperature of 820° C. and tempered at 100 to 350° C. for 3 hours. Then, the hardness and X-ray half value width of each steel were checked. It was found from the test result and studies using, as a reference, published data on these steels (e.g., “Materials” issued by Society of Materials Science, Japan, Vol. 26, No. 280, P26) that the hardness when the X-ray half value width of the martensitic phase is 4 to 4.2° corresponds to a temper hardness of about HRC 51 to 53. Taking account of the fact that the surface carbon concentrations of the carburized, case-hardened steels were adjusted to about 0.7 to 0.9 wt %, the tempering temperature was found to be about 300° C.

It is obvious from the preliminary tests described above that the outermost surface of a tooth flank is tempered and softened by heat generated at the time when the gears come into engagement under high interface pressure so that pitting occurs, and that a 300° C.-temper hardness of HRC 53 or more is necessary, as an index, for obtaining the same level of pitting resistance as that of the carburized quenched gears.

It has also been understood from the comparison between the 300° C.-temper hardness of the carburization-hardened layer of the SCM420 steel which has undergone carburizing/quenching and the 300° C.-temper hardness of the eutectoid carbon steel which has undergone quenching that since virtually no improvement in temper softening resistance can be attained by additions of Cr and Mo, a new alloy design intended for increasing temper softening resistance during low-temperature tempering at about 300° C. is necessary in order to achieve pitting resistance equal to or more than that of the carburized, quenched gears by bright hardening or induction hardening. Also, dispersion of fine cementite particles or the like in the martensitic phase has proved effective as seen from the cases of eutectoid carbon steel (0.82 wt % C) and SUJ2 which were improved in rolling contact fatigue strength.

As a gear design value which provides pitting resistance equal to or higher than the pitting resistance obtained by the carburizing/quenching (interface pressure Pmax=230 kgf/mm² or more) described above, the hardness which can withstand fatigue caused by pulsating shear stress (R=0) which is 0.3 times the value of interface pressure may be set based on the theoretical analysis of Hertz's contact pressure. Its calculated value is approximately HRC 53.4 which coincides with the hardness (HRC=53) obtained from the X-ray half value width of the martensitic phase of the rolling contact surface in which occurrence of pitting was observed in the above-described preliminary test. Since pitting occurs at the time when the temperature of the outermost portion of the rolling contact surface increases to about 300° C. owing to the friction heat generated by rolling/sliding, it has been found that a highly pressure-resistant gear having interface pressure resistance equal to or higher than that of the carburized quenched gears can be developed by setting 300° C.-temper hardness to HRC 54 or more which can withstand Pmax=230 kgf/mm².

As will be described in Example 2, the 300° C.-temper hardness of the martensitic phase of a carbon steel containing 0.1 to 1.0 wt % carbon is described by: HRC=36×{square root}{square root over (C)}(wt %)+20.9

After checking, based on the above hardness, the influences of various alloy elements upon the hardness of the martensitic phase after tempering at 300° C., it has become apparent that the hardness of the martensitic phase after tempering at 300° C. is represented by: HRC=(36×{square root}{square root over (C)}(wt %)+20.9)+4.3×Si(wt %)+7.3×Al (wt %)+3.1×V(wt %)+1.5×Mo(wt %)+1.2×Cr(wt %)×(0.45÷C(wt %))

It should be noted that the coefficient (in the case of Si for instance, this coefficient is 4.3ΔHRC/wt %) proportional to the weight percent of each alloy element of the above equation indicates the temper softening resistance of the alloy element.

In the invention, the content (wt %) of each alloy element constituting the above steels is defined as follows based on the above-described gear materials and thermal treatment designs.

To sum up, there is provided a rolling element according to the invention which is made from a steel material containing at least 0.15 to 0.35 wt % C; further containing either 1.0 to 3.0 wt % Si or 0.35 to 1.5 wt % Al or alternatively, 0.5 to 3.0 wt % (Si+Al); and further containing one or more alloy elements selected from the group consisting of Mn, Ni, Cr, Mo, V, Cu, W, Ti, Nb, B, Zr, Ta, Hf, and Ca, unavoidable impurities such as P, S, N and O, and balance essentially consisting of Fe; the steel material being prepared so as to satisfy the relationship described by: 5≦4.3×Si(wt %)+7.3×Al(wt %)+3.1×V(wt %)+1.5×Mo(wt %)+1.2×Cr(wt %)×(0.45÷C(wt %)), and

which is formed by carburizing the steel material such that the carbon concentration of a carburized surface layer of the steel material is adjusted to 0.6 to 0.9 wt %; quenching the steel material subsequently to the carburization process and then tempering the steel material at 300° C. or less, or alternatively cooling the steel material once after the carburization process and then applying treatments of re-heating hardening and tempering at 300° C. or less to the steel material so that a hardness of HRC 58 or more, more preferably, HRC 60 or more is ensured by the tempering process at 300° C.

While the 300° C. temper-hardness of the carburized layer of an SCM-based carburized, quenched material is usually within the range of from HRC 53 to HRC 56, the 300° C.-temper hardness is set to HRC 58 or more in the invention on the ground that: (i) an improvement in pitting resistance can be clearly observed and (ii) taking account of the fact that the percentage of compactness when a mechanical reduction gear is downsized by one lank is 25 to 30% and the contact fatigue strength of the gear in this case is no less than 1.15 times the contact fatigue strength of the conventional gear (230→265 kgf/mm²), the 300° C.-temper hardness is HRC 58 or more.

An addition of about 1 wt % Al is apparently preferable for improvement of pitting resistance, because 15 to 25% by volume of the residual austenitic phase existing, for example, in the carburized layer of an SCM-based steel is reduced to 10% by volume or less, which has the effect of increasing the hardness of the carburized layer of the surface (ΔHRC=2).

It is also apparently preferable for rolling elements such as gears having higher strength to apply mechanical pressurization treatment such as shot peening or roller burnishing to the tooth flanks, dedenda and tooth bottoms of the gear with the intention of improving the strength of the tooth flanks and the bending strength of the dedenda, whereby a distinct compressive residual stress is generated. It is apparent that the elements to which such treatment is applied are also within the scope of the invention.

The above-described carburization is usually carried out at 900° C. or more. Where Si and Al are contained in high concentration as described earlier, the dual-phase (α+γ) state develops in a reheating condition in the raw material composition part having low carbon content and positioned deeper than the carburized layer, and quenching starts from this condition so that the strength of the inside of the carburized layer decreases. This drawback can be overcome by setting carburized case depth taking account of the distribution of bearing stress and the distribution of bending stress. In addition, it is economically disadvantageous to increase carburized case depth and therefore, the A3 transformation temperature is adjusted by adding the austenite stabilizing elements C, Mn and Ni in combination with the ferrite stabilizing elements Si and Al as described earlier, thereby controlling the temperature of carburization to a typical carburization temperature of 950° C. or less.

In the case of a steel material containing either 1.0 to 3.0 wt % Si or 0.35 to 1.5 wt % Al or alternatively, 0.5 to 3.0 wt % (Si+Al) to increase temper softening resistance, an addition of 3 wt % Si increases the A3 transformation temperature by about 170 degrees (see FIG. 1) and an addition of 1.5 wt % Al also increases it to the same extent, where the carbon content is 0.20 wt %. Therefore, the upper limits of the amounts of Si and Al are set to 3.0 wt % and 1.5 wt %, respectively. According to a third aspect of the invention, Mn and/or Ni is added in the range of 1.0 to 2.5 wt % (Mn+Ni) with the intention of restraining the temperature of quenching by lowering the A3 transformation temperature through proper additions of the austenite stabilizing elements such as C, Mn, Ni and Cu.

Since carbon and nitrogen are extremely effective as an austenite stabilizing element (see FIG. 1), the lower limit of the original carbon content of the steel material is preferably 0.15 wt % from the above viewpoint and the upper limit of the carbon content is preferably 0.35 wt % with which the hardness of the raw material composition part inside the carburized layer after quenching and tempering does not exceed HRC 55. More preferably, the lower limit of the carbon content is 0.2 wt %.

In addition, since nitrogen often reduces the temper softening resistance of Al, it is necessary to prevent penetration of nitride from the carburized or carbonitrided gas atmosphere and, therefore, creation of Al nitrides. In view of this, the N content of the carburized layer is set to at least 0.1 wt %.

According to the invention, there is provided a rolling element which is made from a steel material containing at least 0.15 to 0.35 wt % C; further containing either 1.0 to 3.0 wt % Si or 0.35 to 1.5 wt % Al or alternatively, 0.5 to 3.0 wt % (Si+Al); and further containing one or more alloy elements selected from the group consisting of Mn, Ni, Cr, Mo, V, Cu, W, Ti, Nb, B, Zr, Ta, Hf, and Ca, unavoidable impurities such as P, S, N and O, and balance essentially consisting of Fe; the steel material being prepared so as to satisfy the relationship described by: 5≦4.3×Si(wt %)+7.3×Al(wt %)+3.1×V(wt %)+1.5×Mo(wt %)+1.2×Cr(wt %)×(0.45÷C(wt %)), and

which is formed by carburizing the steel material such that the carbon concentration of a carburized surface layer of the steel material is adjusted to 0.9 to 1.5 wt %; applying treatments of reheating hardening and tempering at 300° C. or less to the steel material after cooling to a temperature equal to or lower than A1 temperature from a state where no cementite precipitates in the surface layer during the carburization process, so that fine cementite particles having a size of 1 μm or less are dispersed within the tempered martensitic phase of the carburized surface layer and a hardness of HRC 60 or more, more preferably, HRC 62 or more is ensured by the tempering process at 300° C.

The reason why the lower limit of the carbon content of the surface area of the carburized layer is set to 0.9 wt % is that eutectoid carbon concentration is markedly decreased by additions of Si and Cr and undissolved cementite is stably formed in an amount of 3% by volume or more when the carbon content is 0.9 wt % or more. The reason why the carbon content is limited to 1.5 wt % or less is that if the carbon content exceeds 1.5 wt %, coarse cementite particles (3 μm or more) will be unavoidably created owing to aggregation of cementite particles, so that there arises the high risk of a decrease in the bending strength of the gear. In addition, for carrying out carburization with a high carbon concentration of 1.5 wt % or more without causing precipitation of coarse cementite particles in the surface layer during the carburization process, it is necessary to increase carburization temperature to about 1100° C. which is practically difficult because of the limitation in terms of equipment.

Since the carburization which provides a surface carbon concentration of 0.9 to 1.5 wt % is carried out in a high carbon potential condition with a carbon activity (ac) of about 1 and is preferably carried out in the high temperature region (1000° C. or more), there is the possibility of precipitation of coarse cementite particles in the surface layer during the carburization, and therefore, carbon potential needs to be controlled with high accuracy. However, it is extremely difficult to control high carbon potential carburization carried out at a temperature of 1000° C. or more. Focussing on the fact that Cr contained in a steel material promotes precipitation of coarse cementite particles, the invention is arranged such that precipitation of cementite is prevented even in high carbon potential carburization, by reducing the amount of Cr to 0.5 wt % or less or by limiting the amount of Cr to no more than 1.4 times the amount of Si.

More precisely speaking, the effects of Mn, Ni, Mo and the like should be taken into account and it is preferable to consider the relationship described by: −0.146×Si(wt %)+0.03×Mn(wt %)−0.024×Ni(wt %)+0.075×Cr(wt %)+0.043×Mo(wt %)+0.133×V(wt %)≦0

Practically, it is preferable to set the amount of Si or (Si+Al) to 1.5 to 2.5 wt % and to limit the amount of Cr to 2.0 wt % or less.

As discussed earlier, the invention is intended for dispersion of fine cementite particles on condition that reheating hardening treatment is applied to the steel and, therefore, the temperature of the reheating hardening treatment is equal to or more than the A1 transformation temperature. In the case where Si and Al are contained in high concentration, the (α+γ) dual phase state develops under the reheating condition within the raw material composition part having low carbon content and positioned deeper than the carburized layer and quenching starts from this condition so that the strength of the inside of the carburized layer decreases as described earlier, however, it is apparent that this problem can be solved by setting the carburized case depth taking account of the distribution of bearing stress and the distribution of bending stress. Since increasing of the carburized case depth is disadvantageous in view of cost, it is preferable to adjust the A3 transformation temperature by controlling the amounts of carbon, Mn and Ni in compliance with the amounts of Si and Al so that the temperature of the reheating hardening process is set to 950° C. or less.

Where the temperature of the reheating hardening process is set to a value as high as 850 to 950° C., it is difficult to fine cementite particles dispersing in an ordinary SCM-based material (0.75 wt % Mn, 1 wt % Cr, 0.15 wt % Mo) so as to have a size of 1 μm or less. Therefore, the steel of the invention contains at least 0.4 wt % or less V which condenses to a significant degree within cementite when austenite and cementite are in the equilibrium state. It should be noted that the distribution coefficient of each alloy element defined by a distribution coefficient KM (KM=the concentration of M element in cementite (wt %)÷the concentration of M element (wt %) in austenite) is given by:

-   -   KV=12.3, KCr=6.4, KMo=3.5, KNi=0.22, KSi, Al≈0

According to the invention, there is provided a rolling element which is made from a steel material containing at least 0.35 to 0.60 wt % C; further containing either 1.0 to 3.0 wt % Si or 0.35 to 1.5 wt % Al or alternatively, 0.5 to 3.0 wt % (Si+Al); and further containing one or more alloy elements selected from the group consisting of Mn, Ni, Cr, Mo, V, Cu, W, Ti, Nb, B, Zr, Ta, Hf, and Ca, unavoidable impurities such as P, S, N and O, and balance essentially consisting of Fe; the steel material being prepared so as to satisfy the relationship described by: 5≦4.3×Si(wt %)+7.3×Al(wt %)+3.1×V(wt %)+1.5×Mo(wt %)+1.2×Cr(wt %)×(0.45÷C(wt %)), and

which is formed by tempering the steel material at 300° C. or less after quenching treatment such as induction hardening so that a hardness of HRC 55 or more is ensured for a hardened surface layer of the steel material by the tempering process at 300° C.

An essential condition of the invention is that the hardness of the steel after tempering at 300° C. subsequent to quenching is HRC 55 or more. To satisfy this condition, the steel material is preferably hardened by quenching so as to have a hardness of about HRC 58 or more and therefore the lower limit of the carbon content is set to approximately 0.35 wt %. Where the temper softening resistance which corresponds to a 300° C.-temper hardness of HRC 53 is ensured by sole additions of Si or Al, it becomes necessary to add 2.5 wt % or more Si or 1.47 wt % or more Al as seen from the foregoing equation, which causes the temperature of quenching to be as high as 900° C. or more (see FIG. 1). It is obviously more preferable for the invention to properly control the amount of carbon which is an extremely effective austenite stabilizing element, thereby restricting an increase in quenching temperature and to set the amount of carbon to 0.43 wt % or more in order to ensure stable hardness after quenching.

A preferable upper limit of the amount of carbon is 0.6 wt % or less when taking account of quenching crack susceptibility at the time of induction hardening. It is understood from simple calculation that where carbon is added within the range of from 0.4 wt % to 0.6 wt %, proper amounts of Si and Al in the case of sole addition are 1.0 wt % or more and 0.6 wt % or more, respectively.

Further, it is apparent that, in order to obtain the substantially same level of contact fatigue strength as the average tooth flank strength of the carburized, quenched gears, the 300° C.-temper hardness is preferably HRC 55 or more, and the upper limit of the amount of carbon is preferably 0.55 wt % when taking account of susceptibility to quenching cracks caused by water or an aqueous quenching liquid used in the induction hardening.

As an effective way of producing a gear member capable of withstanding high interface pressure, the above-described high carbon-concentration carburization is applied to the gear material of the invention such that fine cementite particles having a size of 1 μm or less are dispersed in the surface layer. For preventing quenching cracks occurring during the reheating hardening process or the induction hardening process, it is preferable to utilize an aqueous quenching liquid or quenching oil having a high concentration of a polymer element.

To attain higher contact fatigue strength than that of the above-described carburized quenched gears, the inventors have developed a rolling element such as a gear which is made from a steel material containing at least 0.60 to 1.50 wt % C; further containing either 1.0 to 3.0 wt % Si or 0.35 to 1.5 wt % Al or alternatively, 0.5 to 3.0 wt % (Si+Al); and further containing one or more alloy elements selected from the group consisting of Mn, Ni, Cr, Mo, V, Cu, W, Ti, Nb, B, Zr, Ta, Hf, and Ca, unavoidable impurities such as P, S, N and O, and balance essentially consisting of Fe; the steel material being prepared so as to satisfy the relationship described by: 5≦4.3×Si(wt %)+7.3×Al(wt %)+3.1×V(wt %)+1.5×Mo(wt %)+1.2×Cr(wt %)×(0.45÷C(wt %)), and

which is formed by tempering the steel material at 300° C. or less after quenching treatment such as induction hardening so that a hardness of HRC 58 or more is ensured for a hardened surface layer of the steel material by the tempering process at 300° C.

Since there is no need to dissolve all the cementite in the austenite, the heating temperature of the induction hardening process can be set to a value in the dual phase (austenite and cementite) coexisting region having an A1 transformation temperature of 950° C. or less and the concentration of carbon dissolved in the austenite can be set to a smaller value under this condition. By this arrangement and utilization of a quenching oil or an aqueous polymer quenching liquid as a quenching medium, quenching crack susceptibility can be reduced.

Where fine cementite particles are dispersed by high frequency heating and quenching, undissolved cementite is unlikely to be coarsened because the dispersion is carried out in a short time (within several minutes) by rapid heating, and, therefore, the addition of V is not indispensable. However, the addition of V is useful from the viewpoint of further fining the structure prior to the induction hardening process and the additions of Cr, V, Mo and Mn is useful from the same view point.

In addition, in the case of a rolling element such as a gear used under higher interface pressure, it can be assumed that the rolling contact surface is exposed to higher temperature, and V exhibits remarkable temper softening resistance ΔHRC (350° C.:4.6, 400° C.:6.1, 450° C.:9.2). Therefore, the amount of V is set to 0.05 wt % or more with which the effect of V becomes conspicuous. The upper limit of the amount of V is set to 0.4 wt % for the reason that the effect of V can be effectively utilized when it is added in this amount in the case where the maximum quenching temperature is 950° C.

Although it is favorable to positively add Mo, because Mo exerts remarkable temper softening resistance (350 ° C.:2.4, 400° C.:3.23, 450° C.:4.9) in the higher temperature region, the upper limit of the amount of Mo is set to 0.35 wt % from the economical viewpoint.

Where the additions of large amounts of Si and Al cause graphite precipitation in the process of manufacturing the steel material or in the thermal treatment of the invention, there is a likelihood of a significant decrease in strength. Therefore, it is apparently preferable for the invention to add 0.2 to 0.5 wt % Cr which significantly stabilizes at least cementite and prevents graphitization.

The rolling element such as a gear contains one or more elements selected from 0.3 to 1.5 wt % Mn, up to 0.35 wt % Mo and 0.0005 to 0.005 wt % B, on the ground that the induction hardened steel material is not required to have high hardenability since the induction hardening treatment is a heating process in which heat is applied to only the part to be quench-hardened and its neighborhood, unlike furnace heating.

The present inventors have already reported in Japanese Patent Application No. 2002-135274 that considerably high toughness can be achieved by the coexistence of Al in the aforesaid amount and 0.3 to 2.5 wt % Ni and excellent Charpy impact characteristic can be obtained by it, particularly, in high-hardness martensitic structures containing 0.6 wt % carbon and 1.2 wt % carbon. Ni contributes to a dramatic improvement in the impact load resistance of a gear and is therefore apparently beneficial as a gear material. Since the addition of Ni increases the cost of the steel material, the amount of Ni to be added is limited to 1.5 wt % or less in the invention.

If the carburization temperature, reheating temperature and induction hardening temperature become too high in the invention, there may arise the problem of coarsening austenite crystal grains. In this case, it is obviously desirable to add the known elements called “crystal grain fining elements” such as Ti, Nb, Zr, Ta and Hf in an amount ranging from 0.005 to 0.2 wt %.

There will be hereinafter summarized the function of each of the alloy elements constituting the above-described rolling elements of the invention.

Si: 0.8 to 3.0 wt %

Si is an element which significantly enhances temper softening resistance in tempering at a low temperature of 300° C. to 350° C. The mechanism of enhancing temper softening resistance is such that softening is prevented by further stabilizing ε carbides which precipitate at low temperature and causing cementite precipitation to occur in a higher temperature region.

(1) Induction Hardened Gears

Since the softening resistance ΔHRC of Si per 1 wt % in tempering at 300° C. is 4.3 and the 300° C.-temper base hardness obtained from 0.6 wt % carbon is HRC 48.8, the amount of Si for ensuring a 300° C.-temper hardness of HRC 53 is about 1.0 wt % and the amount of Si when it coexists with 0.15 wt % Al is about 0.8 wt %. On this ground, the lower limit of the amount of Si is set to 0.8 wt % and more preferably to 1.5 wt % to enhance its function.

While the upper limit of the amount of Si is set to 3.0 wt % in order that the Ac3 transformation temperature does not exceed 900° C., and the quenching temperature is prevented from increasing more than is necessary where the amount of carbon is within the aforesaid range of from 0.35 wt % to 0.6 wt %, the upper limit of the amount of Si is preferably 2.5 wt % or less in the case where the lower limit of the amount of carbon contained in the steel material for induction hardened gears is 0.4 wt %.

(2) Carburized and Quenched Gears

The range of Si content noted earlier can be substantially suitably applied to the upper and lower limits of the amount of Si to be added for ensuring a 300° C.-temper hardness of HRC 60 in the process in which carburization is applied to the surface of the rolling element such as a gear to increase its surface carbon content to 0.6 to 0.9 wt % and then, quenching and tempering at 300° C. or less are carried out.

Also, the range of Si content noted earlier can be substantially suitably applied to the upper and lower limits of the amount of Si added for ensuring at least a 300° C.-temper hardness of HRC 62 or more in the process in which carburization is applied to the surface of the rolling element such as a gear, thereby increasing the surface carbon content to 0.9 to 1.5 wt % and then, reheating hardening treatment is applied so as to disperse fine cementite particles in the rolling contact surface, followed by tempering at 300° C. or less.

To increase the surface carbon content of the rolling element to 0.9 to 1.5 wt % without precipitating cementite in the surface layer during the carburization process, the carbon activity in the carburization process carried out at a high temperature of 930 to 1100° C. needs to be increased. In this case, precipitation of coarse cementite (3 to 15 μm) (excessive carburization) mainly due to the addition of Cr element is likely to occur, resulting in a considerable decrease in the strength of the gear. In the invention, this problem is solved by positively adding Si which prevents excessive carburization while the amount of Cr is limited so as not to exceed 1.4 times the amount of Si, and, more precisely, by using a steel material which meets the relationship described by −0.146×Si(wt %)+0.03×Mn(wt %)−0.024×Ni(wt %)+0.075×Cr(wt %)+0.043×Mo(wt %)+0.133×V(wt %)≦0

Where the above steel material is used, the vacuum carburization method wherein carburization is carried out with a carbon activity of 1 can be employed. This method is extremely beneficial for producing a rolling element such as a gear, because high temperature carburization at 1100° C. or less can be carried out at low cost, and its function of inhibiting coarse cementite precipitation is advantageous for increasing the strength of the rolling element such as a gear.

Since Al has the strong deoxidization function as well as the strong function of expelling P and S from the grain boundary, P and S being impurities contained in steel, Al is useful for cleaning steel materials. Further, it has been confirmed that Al enhances temper softening resistance more than Si does (ΔHRC=7.3) in low temperature tempering. In the light of the above facts, the invention is designed such that where Al is solely added; the amount of Al is 0.35 to 1.5 wt % and where part of Si is replaced with 0.15 to 1.5 wt % Al, the total amount of Si and Al is 0.5 to 3.0 wt %. Since Al is a stronger ferrite stabilizing element than Si as noted earlier and has the function of increasing the Ac3 temperature about 1.6 times higher than Si does, the maximum amount of Al is set to 1.5 wt % (=2.5 wt %/1.6) or less.

It has been reported in Japanese Patent Application No. 2002-135274 that remarkable toughness can be achieved by the coexistence of the above amount of Al and 0.3 to 2.5 wt % Ni, and excellent Charpy impact characteristic can be achieved, particularly, in high-hardness martensitic structures containing 0.6 wt % carbon and 1.2 wt % carbon. Ni contributes to a remarkable improvement in the impact load resistance of a gear and is therefore apparently beneficial as a gear material. Since the addition of Ni increases the cost of the steel material, the amount of Ni is limited to 1.5 wt % or less in the invention.

Since Mn not only exhibits remarkable desulfurization but also stabilizes austenite as noted earlier and further has the beneficial effect of improving the hardenability of steel, Mn is added in a proper amount according to purposes. Taking account of the fact that, in a steel containing 0.35 to 0.6 wt % carbon, austenite is satisfactorily stabilized by carbon, the lower limit of the amount of Mn is set to 0.3 wt %.

Mo is a useful element as it improves the hardenability of steel and restrains temper brittleness, and therefore it is desirable for the invention to add Mo in an amount of 0.35 wt % or less which is at the same level as that of ordinary case-hardened steels.

In cases where the tooth flanks of a gear are quench-hardened by induction hardening, only the surface layer which has been heated to a temperature equal to or higher than the Ac3 transformation temperature by high-frequency heating may be quench-hardened, and therefore the gear material is not required to have hardenability (DI value) higher than the hardenability (3.0 inches) of the ordinary carbon steel level. This means that inexpensive steel materials can be employed. In the invention, the addition of Mn and Cr is further suppressed and the addition of alloy elements such as Si, Al, Ni, Mo and V is controlled to obtain a DI value of 3.0 inches or less.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a phase diagram showing the effects of various alloy elements which constitute Fe3Si.

FIGS. 2(a) and 2(b) illustrate a test specimen used for a roller pitting test.

FIG. 3 is a graph showing the result of a preliminary test for checking roller pitting resistance.

FIG. 4 is a graph showing the comparison between the measured values and calculated values of temper hardness.

FIG. 5 is a graph (1) showing the pitting resistance of steels according to the invention.

FIGS. 6(a) and 6(b) show patterns for vacuum carburization quenching treatment.

FIG. 7 is a graph (2) showing the pitting resistance of steels according to the invention.

FIG. 8 is a graph (3) showing the pitting resistance of steels according to the invention.

FIGS. 9(a) and 9(b) are photographs of the metallographic structures of the carburized layers of test specimens No. 5 and No. G2.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring now to the accompanying drawings, rolling elements of the invention will be hereinafter described according to preferred embodiments of the invention.

EXAMPLE 1

The Pitting Resistance of Quenched, Tempered Carbon Steel and Carburized, Quenched, Case-Hardened Steel

(Preliminary Test)

In this example, a roller pitting test was conducted with the test specimen shown in FIG. 2 and the pitting resistance of various quenched, tempered carbon steels and carburized, quenched, case-hardened steels was checked to investigate the rolling contact fatigue strength of the tooth flanks of gears. Table 1 shows the chemical compositions of the various carbon steels and case-hardened steels used in this example. These steel materials were respectively shaped into the small roller test specimen shown in FIG. 2(a) and the test specimens No. 1, 2 and 4 were subjected to water quenching after heating at 820° C. for 30 minutes, and then tempered at 160° C. for 3 hours, followed by testing. The specimen No. 3 was quench-hardened, at its rolling contact surface, using a 40 kHz high-frequency power source after thermal refining and then subjected to tempering similarly to the above specimens. No. 5 was cooled to 850° C. after carburization (carbon potential=0.8) at 930° C. for 5 hours. Then, it was kept at 850° C. for 30 minutes and quenched by a quenching oil having a temperature of 60° C., followed by the same tempering treatment as described above. TABLE 1 C Si Mn Ni Cr Mo NOTE No. 1 0.55 0.23 0.71 S55C No. 2 0.77 0.21 0.74 EUTECTOID CARBON STEEL (1) No. 3 0.85 0.22 0.81 EUTECTOID CARBON STEEL (2) No. 4 0.98 0.27 0.48 1.47 SUJ2 No. 5 0.19 0.22 0.75 0.97 0.15 SCM420H

The large roller test specimen shown in FIG. 2(b) was prepared by applying water quenching to the SUJ2 material of No. 4 after heating at 820° C. for 30 minutes and then tempering it at 160° C. for 3 hours. The roller pitting test was carried out in such a way that the small and large (loaded) rollers were rotated at speeds of 1050 rpm and 292 rpm respectively, while being lubricated with #30 engine oil having a temperature of 70° C., and a load is imposed on the rollers with a slip ratio of 40% and interface pressures ranging from 375 to 220 kgf/mm².

FIG. 3 collectively shows the number of repetitions which causes occurrence of pitting under each interface pressure. In FIG. 3, a lifetime line (indicated by solid line) is shown, which is formed by connecting the respective minimum numbers of repetitions of the reference carburized case-hardened steel tested under the various interface pressures. If the interface pressure when the number of repetitions which causes occurrence of pitting is 10⁷ times was defined as rolling contact fatigue strength, the pitting resistance was found to be about 210 kgf/mm². When checking the pitting resistance of each test specimen in the same way, it was found that No. 1=175 kgf/mm², No. 2=240 kgf/mm², No. 3=260 kgf/mm², and No. 4=260 kgf/mm². It was further found that the pitting resistance of the carburized case-hardened steels varied to a somewhat large extent because of intergranular oxidation which had occurred during the carburization of the rolling contact surface, the presence of a slack quenched layer, and a large amount of residual austenite. It was found from the comparison in terms of the average number of repetitions which causes pitting that the pitting strengths of the carburized case-hardened steels do not differ from that of the test specimen No. 2.

The X-ray half value width of the martensitic phase of the rolling contact surface of each test specimen in which pitting had occurred under an interface pressure of 250 kgf/mm² was checked. As a result, it was found that No. 1=3.6 to 4.0°, No. 2=4 to 4.2°, No. 3=4.2 to 4.4°, No. 4=4.3 to 4.6° and No. 5=4 to 4.2°.

Further, the test specimens Nos. 1 to 5 which had undergone the above-described thermal treatment were tempered at 250 to 350° C. for 3 hours and then, the X-ray half value width of the rolling contact surface of each test specimen in which pitting had occurred was checked. As a result, the half value width of each specimen under the above condition was found to be coincident with the half value width when tempering was carried out at 300° C. It was also found to be substantially coincident with the relationship between the temper hardnesses and half value widths of carbon steels having various carbon concentrations which was reported in “Material” Vol. 26, No. 280, P26.

EXAMPLE 2

Checking of Temper Softening Resistance

Table 2 shows the alloy compositions employed in this example. Thermal treatment was carried out in such a way that after heated at 810 to 870° C. for 30 minutes, each test specimen was subjected to water cooling and then tempering at 300° C. or 350° C. for 3 hours. Thereafter, the Rockwell hardness HRC of each test specimen was checked and the effect of the addition of each alloy element on the hardness was analyzed. TABLE 2 TPNo. C Si Al Mn Ni Cr Mo V B No. 6 0.45 1.45 0.46 1.49 0.52 0.14 0.0018 No. 7 0.49 1.45 0.46 1.01 1.03 0.15 0.0019 No. 8 0.47 0.31 0.46 2.01 1.03 0.15 0.0019 No. 9 0.49 0.29 0.45 1.5 1.49 0.23 0.0019 No. 10 0.36 1.77 0.6 0.62 0.11 0.0026 No. 11 0.45 0.95 0.66 0.01 1.29 0.5 0.0029 No. 12 0.39 0.93 1.02 0.08 0.97 0.95 0.5 No. 13 0.43 0.26 0.44 1.01 0.48 0.001 No. 14 0.47 0.25 0.4 1.01 1.05 0.0018 No. 15 0.46 1.5 0.4 1 0.51 0.002 No. 16 0.45 0.24 0.4 1.02 0.48 0.31 0.0011 No. 17 0.45 1.46 0.39 0.96 0.98 0.001 No. 18 0.41 0.25 0.35 1 0.49 0.0017 No. 19 0.52 2.3 0.57 0.11 No. 20 0.98 0.27 0.48 1.47 No. 21 0.55 0.23 0.71 No. 22 0.77 0.21 0.74 No. 23 0.45 0.21 1.26 0.53 1.51 0.21 No. 24 0.6 0.25 0.97 0.93 0.98 1.04 0.35

In a preliminary experiment, the hardness of a carbon steel containing 0.1 to 1.0 wt % carbon and 0.3 to 0.9 wt % Mn was checked to be utilized as base data for the analysis of the effect of each alloy element. As a result, it was found that the hardness of this steel was approximated by the following equations. HRC=34×{square root}{square root over (C)}(wt %)+26.5(tempering temperature=250° C.) HRC=36×{square root}{square root over (C)}(wt %)+20.9(tempering temperature=300° C.) HRC=38×{square root}{square root over (C)}(wt %)+15.3(tempering temperature=350° C.)

After analyzing the effect of each alloy element based on the hardnesses of the carbon steels noted above, it was found that the temper softening resistance ΔHRC in the case of tempering at 300° C. for instance could be described by the following equation. ΔHRC=4.3×Si(wt %)+7.3×Al(wt %)+1.2×Cr(wt %)×(0.45÷C(wt %))+1.5×Mo(wt %)+3.1×V(wt %)

It was found from this result that Al exerted temper softening resistance 1.7 times higher than that of Si and was therefore extremely effective as an element for improving rolling contact strength.

FIG. 4 shows the degree of coincidence of the temper hardness obtained from the result of the above analysis with the temper hardness obtained from actual measurements. It will be understood from FIG. 4 that temper hardness can be accurately estimated with the variation range of HRC±1. The 300° C.-temper hardness of the carburized layer (0.8 wt % carbon) of SCM420 (No. 5) of Example 1 is indicated by mark ⋆ in FIG. 4 and well coincident with the calculated value.

EXAMPLE 3

An Improvement in Pitting Resistance by Use of Steel Materials Having Excellent Temper Softening Resistance 1

Table 3 shows the alloy components of the steel materials used in this example. The test specimens No. P1 to No. P10 were subjected to tempering at 160° C. for 3 hours subsequently to quenching at 850 to 920° C., whereas the test specimens No. 11 and No. 12 were subjected to induction hardening under the same high frequency heating condition as in Example 1. A roller pitting test was conducted on these test specimens. TABLE 3 300° C. C Si Al Mn Ni Cr Mo V B calculated HRC No. P1 0.34 0.21 1.47 1.17 0.17 0.11 53.96 No. P2 0.39 1.49 0.49 0.51 0.34 0.05 53.91 No. P3 0.41 1.51 0.72 0.32 0.15 51.09 No. P4 0.41 1.5 0.71 0.32 0.16 0.3 51.99 No. P5 0.45 0.18 1.26 0.53 0.5 0.21 55.94 No. P6 0.55 1.51 0.71 0.15 0.16 54.48 No. P7 0.61 1.21 0.75 0.14 54.34 No. P8 0.62 0.21 1.24 0.53 0.12 59.31 No. P9 0.45 1.02 1.26 0.49 0.12 58.78 No. P10 0.61 0.25 1.47 0.93 0.98 1.04 0.35 62.27 No. P11 0.83 1.01 0.31 0.55 0.96 0.38 62.11 No. P12 1.21 0.2 0.52 0.52 1.01 0.51 0.4 66.62

The test for checking pitting resistance was carried out under substantially the same condition as in Example 1 and the test result is shown in FIG. 5. In FIG. 5, the pitting occurrence line obtained in Example 1 is indicated by solid line and the pitting occurrence line obtained in Example 3 is indicated by broken line.

It was found from the above result that the pitting resistance of the rolling contact surface can be dramatically improved by the sole addition of Al or Si or the combined addition of Al and Si and found from the comparison between the test specimens No. P3, P4, P11 and P12 that the pitting resistance of the rolling contact surface can be dramatically improved by the addition of V.

A remarkable improvement in pitting resistance was observed in the test specimens No. 11 and No. 12 which had been induction hardened such that fine cementite particles were dispersed in the martensitic phase of the rolling contact surface.

Table 3 shows the 300° C. temper hardness obtained from the calculation and it will be understood that this temper hardness has good conformity to the interface pressure at which pitting occurs after 10⁷ times repetitions, the interface pressure being calculated from this temper hardness.

EXAMPLE 4

An Improvement in Pitting Resistance by Use of Steel Materials Having Excellent Temper Softening Resistance 2

This example is intended to increase interface pressure strength by carburizing and quenching treatment. Table 4 shows the alloy components of the test specimens. Two kinds of carburizing/quenching treatments as shown in FIGS. 6(a) and 6(b) were applied. The treatment shown in FIG. 6(a) is vacuum carburization carried out at 950° C. (with the intention of achieving a carbon concentration of 0.8 wt % in the carburized layer) with methane gas free from N2 gas and the treatment shown in FIG. 6(b) is carried out at 1,020° C. (with the intention of achieving a carbon concentration of 1.3 wt % in the carburized layer). The test specimens for the high-carbon concentration carburization (1.3 wt % C) were subjected to quenching and tempering after reheating at 900° C. for 30 minutes. TABLE 4 C C Si Al Mn Ni Cr Mo V B No. 1 0.19 0.22 0.75 0.97 0.15 No. G1 0.22 0.83 0.72 0.96 0.16 No. G2 0.21 1.48 1.18 0.45 0.21 0.41 No. G3 0.26 0.81 0.37 1.21 0.49 0.52 0.19 No. G4 0.26 0.21 1.01 1.09 1.51 0.15 0.37 No. G5 0.34 0.21 1.47 1.17 0.17 0.11 No. G6 0.22 0.24 0.99 1.29 1.01 1.02 0.36

A roller pitting test was made under the same condition as in Example 1. The test result is shown in FIGS. 7 and 8. FIG. 7 shows the result of the test using the test specimens which were subjected to carburizing/quenching/tempering treatment intended for a carburized surface layer containing 0.8 wt % C. Compared with the result of the reference specimen No. 5, No. G3 to No. G6 containing Al have presented an obvious improvement. It has been proved by the results of No. G2 and No. G3 that the sole addition of Si definitely leads to an improvement where the amount of Si is approximately 1.0 wt % or more.

FIG. 8 shows the result of a pitting test conducted on the test specimens No. 5, No. G2 and No. G4 which were subjected to reheating/quenching/tempering treatment such that the carburized surface layer had a carbon content of 1.3 wt % and such that cementite particles were dispersed in the tempered martensitic phase of the rolling contact surface. Compared with the reference specimen No. 5, they were significantly improved in interface pressure strength.

FIGS. 9(a) and 9(b) show structural photographs of the carburized surface layers of the test specimens No. 5 and No. G2. In No. G2, the cementite particles dispersing in the martensitic phase and, therefore, the martensite were fined by the addition of V, which obviously contributed to the significant improvement in interface pressure strength. 

1. A rolling element which is made from a steel material containing at least 0.35 to 0.60 wt % C; further containing either 1.0 to 3.0 wt % Si or 0.35 to 1.5 wt % Al or alternatively, 0.5 to 3.0 wt % (Si+Al); and further containing one or more alloy elements selected from the group consisting of Mn, Ni, Cr, Mo, V, Cu, W, Ti, Nb, B, Zr, Ta, Hf, and Ca, unavoidable impurities such as P, S, N and O, and balance essentially consisting of Fe; said steel material being prepared so as to satisfy the relationship described by: 5≦4.3×Si(wt %)+7.3×Al(wt %)+3.1×V(wt %)+1.5×Mo(wt %)+1.2×Cr(wt %)×(0.45÷C(wt %)), and which is formed by tempering said steel material at 300° C. or less after quenching treatment such as induction hardening so that a hardness of HRC 55 or more is ensured for a hardened surface layer of the steel material by the tempering process at 300° C.
 2. The rolling element according to claim 1, wherein said steel material contains one or more elements selected from the group consisting of 0.3 to 1.5 wt % Mn, 0.5 wt % or less Cr, 0.35 wt % or less Mo, 0.4 wt % or less V and 0.0005 to 0.005 wt % B.
 3. The rolling element according to claim 1, wherein said steel material having an Al content of 0.3 wt % or more contains 0.3 to 1.5 wt % Ni.
 4. The rolling element according to claim 2, wherein said steel material having an Al content of 0.3 wt % or more contains 0.3 to 1.5 wt % Ni.
 5. A rolling element which is made from a steel material containing at least 0.60 to 1.50 wt % C; further containing either 1.0 to 3.0 wt % Si or 0.35 to 1.5 wt % Al or alternatively, 0.5 to 3.0 wt % (Si+Al); and further containing one or more alloy elements selected from the group consisting of Mn, Ni, Cr, Mo, V, Cu, W, Ti, Nb, B, Zr, Ta, Hf, and Ca, unavoidable impurities such as P, S, N and O, and balance essentially consisting of Fe; said steel material being prepared so as to satisfy the relationship described by: 5≦4.3×Si(wt %)+7.3×Al(wt %)+3.1×V(wt %)+1.5×Mo(wt %)+1.2×Cr(wt %)×(0.45÷C(wt %)), and which is formed by tempering said steel material at 300° C. or less after quenching treatment such as induction hardening so that a hardness of HRC 58 or more is ensured for a hardened surface layer of the steel material by the tempering process at 300° C.
 6. The rolling element according to claim 5, wherein said steel material contains one or more elements selected from the group consisting of 0.3 to 1.5 wt % Mn, 0.5 wt % or less Cr, 0.35 wt % or less Mo, 0.4 wt % or less V and 0.0005 to 0.005 wt % B.
 7. The rolling element according to claim 5, wherein said steel material having an Al content of 0.3 wt % or more contains 0.3 to 1.5 wt % Ni.
 8. The rolling element according to claim 6, wherein said steel material having an Al content of 0.3 wt % or more contains 0.3 to 1.5 wt % Ni. 